>>8
Dell Dimension XPS T450 Desktop (450-MHz Pentium III, 64 MB RAM, 6.4 GB hard drive)
I'm assuming those are the specs (a quick Google search brought me to a page on Amazon's Web site with those specs). Pretty old machine. I've got an IMB Aptiva from around that era, it's a 350 MHZ AMD K-6 2, same amount of RAM. Purchased back in April 1998 or so.
But anyway, since you said that you cannot get the installer disc to run and you've changed the boot order (I'm assuming you did this through the BIOS), it could be a corrupted disc or something along those lines. Have you burned the disc yourself? Or is this an official CD from Ubuntu? It could be a DVD data format disc, and you're trying to have the installer disc run from a drive that can only read CD format.
If you know for certain that it's the right format and everything, it could be corrupted. If you burned it yourself, sometimes when you burn a disc at too fast a speed, you run into problems. Try burning a new one at half the speed this time (for instance, you burned the CD at 48x, try 24x this time, or even 16x).
Try those things and if you're still having problems, report back here. Also, when doing so, try to be as detailed as possible in your postings. Thanks, and good luck.